Hitherto, electroluminescent displays, hereinafter referred to as LED, have been used as a light emission type electron displaying device. As the device for constituting ELD, inorganic electroluminescent devices and organic electroluminescent devices, hereinafter referred to as organic EL device, are cited. The inorganic electroluminescent devices have been used as a flat light source. However, alternative high voltage is necessary for driving the light emission device. The organic EL device is constituted by a light emission layer containing a light emission compound placed between a cathode and an anode, which is a device emitting light by utilizing the light emission (fluorescence and phosphorescence) caused by quenching of an exciton generated by recombination of electrons and positive holes injected to the light emission layer, and the light can be emitted by applying voltage about several volts to several tens volts. The device is noted from the viewpoint of space saving and portability since which has wide viewing field and high visibility because the device is self light emission, type and is thin layer complete solid device.
However, development of an organic EL device capable of emitting light having higher brightness with higher efficiency and lower power consumption is demanded for practical applying of the organic EL element in the future.
Japanese Patent No. 3,039,796 describes that the improvement in the brightness of emitting light and prolongation of lifetime of the device can be attained by a stilbene derivative, distyrylarylene derivative or tristyrylarylene each doped with a slight amount of fluorescent substance.
Moreover, devices having an organic light emission layer composed of 8-hydroxyquinoline aluminum complex as host compound doped with a slight amount of fluorescent substance (cf. JP-A S63-264692 for example), and devices having an organic light emission layer composed of 8-hydroxyquinoline aluminum complex as host compound doped with a quinacridone type dye (cf. JP-A H03-255190, for example), have been known.
When light emission from the singlet excited state is used as above-mentioned, the ratio of generation ratio of the singlet excitons and the triplet excitons is 1:3. Therefore, the limit of externally taking out efficiency (ηext) of light is 5% because the generation probability of the light emission exciton species is 25% and the taking out efficiency of light is 20%.
Besides, study on the materials emitting phosphorescence at room temperature is accelerated after disclosure of the report by Princeton University as to an organic device using light emission from the triplet excited state; cf. M. A. Baldo et al., Nature, 395, pp. 151 to 154 (1998).
Such the technology is also disclosed in, for example, M. A. Baldo et al., Nature, 403, 17, pp. 750 to 753 (2000) and U.S. Pat. No. 6,097,147. When the triplet excited state is utilized, the upper limit of internal quantum efficiency becomes 100%. Therefore, the light emission efficiency theoretically becomes 4 times of that of utilization of singlet excited state and a probability of obtaining performance almost the same of a cathode ray tube is caused so that such the device is noted as the use for illuminating device.
For example, many compounds, principally heavy metal complexes such as iridium complexes, are synthesized and investigated; cf. S. Lamansky et al., J. Am. Chem. Soc., 123, p. 4304 (2001).
In the above M. A. Baldo et al., Nature, 403, 17, pp. 750 to 753 (2000), the investigation is carried out using tris(2-phenylpridine)iridium as the dopant.
Other than the above, M. E. Tompson et al. investigate L2Ir(acac) such as (ppy)2Ir(acac) as the dopant; cf. The 10th International Workshop on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), and Moon-Jae Youn. Og, Tetsuo Tsutsui investigate using tris(2-(ptolyl)pyridine)iridium (Ir(ptpy)) or tris(benzo[h]quinoline)iridium (Irbzq)3) as the dopant; cf. The 10th International Workshop on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu). These metal complexes are commonly called as ortho-metalized iridium complex.
In the above S. Lamansky et al., J. Am. Chem. Soc., 123, p. 4304 (2001), it is tried to make devices using various iridium complexes.
In the 10th International Workshop on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), Ike et al. uses a hole transfer compound as the host compound of a phosphorescent compound for obtaining high light emission efficiency. M. E. Tompson et al. uses various electron transfer materials doped with novel iridium complexes as the host of the phosphorescent compound.
Ortho-metal complexes having platinum atom as the central metal instead of iridium are also noted. As to such the kind of complexes, many examples are known, which are characterized by the ligand; cf. Patent publications 1 to 5 and Non-patent Publication 1, for example.
In the any cases, the brightness of emitted light and the light emission efficiency of the light emission device are considerably improved compared with conventional devices because the emitted light is originated from phosphorescence. However, there is a problem that the lifetime is shorter than that of the conventional devices. As above-mentioned, it is present state that the phosphorescent light emission material has not properties suitable for practical use because shortening in the wavelength of the emitted light and improving in the lifetime are difficult in such the device.
Regarding the shortening of the wavelength of emitting light, it is known to introduce an electron attractive group such as fluorine atom, trifluoromethyl group and cyano group as a substituent to phenylpyridine and to introduce a ligand such as picolinic acid and a pyrazabole type ligand; for example, cf. Patent Publications 6 to 10 and Non-patent Publications 1 to 4. By such the ligand, the wavelength of emitting light can be shortened so that a device capable of emitting blue light with high efficiency can be obtained but the light emission lifetime of the device is considerably shortened. Therefore, improvement of such the trade-off is demanded.    Patent Publication 1: JP-A 2002-332291    Patent Publication 2: JP-A 2002-332292    Patent Publication 3: JP-A 2002-338588    Patent Publication 4: JP-A 2002-226495    Patent Publication 5: JP-A 2002-234894    Patent Publication 6: WO 02/15645    Patent Publication 7: JP-A 2003-123982    Patent Publication 8: JP-A 2002-117978    Patent Publication 9: JP-A 2003-146996    Patent Publication 10: WO 04/016711    Non-patent Publication 1: Inorganic Chemistry, 41, 12, pp. 3055-3066 (2002)    Non-patent Publication 2: Applied physics Letters, 79, p. 2082 (2001)    Non-patent Publication 3: Applied physics Letters, 83, p. 3818 (2003)    Non-patent Publication 4: New Journal of Chemistry, 26, p. 1171 (2002)